A homeobox gene goosecoid, specifically expressed in the dorsal blastopore lip was isolated. Goosecoid, is particularly interesting since it is: 1) directly activated by activin (a mesoderm-inducing growth factor), 2) specifically expressed in the mesoderm of dorsal lip region, 3) only expressed during the gastrulation process, and 4) capable of inducing a secondary axis when injected into a blastomere of an early embryo. These properties of the goosecoid homeobox gene suggest that the gene plays fundamental regulatory roles during gastrulation. Expression pattern of goosecoid will be studied using both in situ hybridization and antibody staining techniques. In order to understand the function of the goosecoid homeodomain protein, various expression constructs will be microinjected into an individual blastomere of embryos to analyze the cell fate change brought by the expression of goosecoid. Attempts will be made to obtain loss-of-function, gain-of-function, and dominant-negative mutants by microinjecting antibodies, sense and antisense RNAs and oligonucleotides. In order to understand the activin-mediated signal transduction pathway, goosecoid promoter reporter gene constructs will be microinjected into embryos and the activin responsive element in the goosecoid promoter identified. Thus, the identification and characterization of the molecules mediating the signal transduction can be accomplished. Isolation of the goosecoid target genes will be attempted using differential probes obtained from goosecoid microinjected embryos.